1. Field of the Invention
The present invention relates generally to combined communication signals and more particularly to combined CATV and DBS signals.
2. Description of the Related Art
A signal-band diagram 20 of FIG. 1 defines frequencies that are associated with a variety of communication services that deliver communication signals to consumers. In the oldest of these communication services, off-air television and frequency modulation signals are typically received through a consumer antenna. The majority of these off-air signals are arranged in a plurality of signal bands that range from 54 to 216 MHz and are shown in broken lines as bands 22 in FIG. 1.
In a second one of these services, communication signals are provided by a wireless cable system in which signals are directed from a service provider's antenna to a plurality of subscriber antennas. The signals are sent over the multipoint distribution service (MDS) frequency band (2150-2162 MHz) and the multichannel multipoint distribution service (MMDS) frequency band (2500-2686 MHz). Signals in these wireless cable bands are typically downconverted at subscriber dwellings by low noise block downconverters (LNB's) to form MDS and MMDS intermediate-frequency bands that respectively span 116-128 MHz and 222-408 MHz. These are respectively shown in broken lines as bands 24 and 26 in FIG. 1.
Consumers are also offered the alternative of cable television (CATV) in which hard cables deliver television and frequency modulation signals to consumer locations over a signal band of 54-648 MHz.
The communication signals provided by these three communication systems were initially limited to television and frequency modulation signals. Consumers are now being offered, however, an increasing list of other communication options. For example, a communication service can carry telephony signals and also operate as an internet service provider (ISP) who provides access to the internet. It was also initially envisioned that signals were only downlinked to consumers but some of these communication services have now become two-way streets in which consumers uplink data signals (e.g., signals associated with the activities of pay-for-view, banking, home shopping, medical alarm and fire/security).
To provide for both uplink and downlink processes, a CATV signal band is generally defined to span 5-750 MHz and includes an uplink signal band of 5-30 MHz for consumer uplinking of data signals and a downlink signal band of 50-750 MHz for provider downlinking of communication signals. The CATV signal band is shown in FIG. 1 as the band 28.
Consumers can presently also choose to receive communication signals from a direct broadcast satellite (DBS) system. In this system, satellites radiate microwave signal beams in C-band and Ku-band frequencies. Upon direct receipt at a consumer antenna, these satellite signals are initially downconverted to an intermediate-frequency signal band of 950-1450 MHz before further downconversion and detection at a lower detection frequency.
To increase their number, DBS channels are typically grouped in two sets which each basically span the same DBS band but each channel of one set is centered over a boundary between adjacent channels of the other set. In order to further isolate the sets, they are transmitted with different signal polarizations (e.g., vertical and horizontal polarizations or left-hand and right-hand circular polarizations). Once the transmitted signals are detected in an antenna, the polarization isolation is lost and the two intermediate-frequency signal bands must either be carried on separate transmission lines (e.g., coaxial cables) or carried on a common transmission line after one channel set has been frequency offset from the other.
In either case, DBS signals present distribution problems. If the two DBS channel sets are carried away from an antenna on separate cables, these cables must be continued all the way to the consumer's equipment (e.g., a television set). This is especially difficult and costly in multiuser installations because it means that all link portions of a distribution system must be formed with pairs of cables.
If the intermediate-frequency channel sets are, instead, frequency offset, they typically form a lower DBS signal band of 950-1450 MHz and an upper DBS signal band of 1525-2025 MHz. These are respectively shown in FIG. 1 as DBS signal bands 30 and 32. These two signal bands can be carried over a single-cable distribution system because of their frequency separation. Because the CATV signal band 28 of FIG. 1 also occupies a different frequency band, it can be carried over the same single-cable distribution system.
Television sets (and other communication devices) typically cannot receive CATV and DBS signals directly and they are, therefore, complimented by interface modules commonly referred to as "set tops". More precisely, these interfaces are generically termed integrated receiver decoders (IRD). Because they are generally configured to only accept the lower DBS signal band 30, they are not compatible with a single-cable distribution system that carries both DBS signal bands 30 and 32.
To summarize, communication signals for consumers are generally grouped into a CATV signal band (28 in FIG. 1) and lower and upper DBS signal bands (30 and 32 in FIG. 1). They provide desirable communication services (e.g., television, internet and telephony) and facilitate downlink and uplink processes. However, these communication signal bands are not currently compatible with single-cable distribution systems.
Conventional efforts to resolve this incompatibility have generally involved the use of mechanical relay switches or IRD's which have been modified to receive the lower and upper DBS signal bands. The former effort accepts the low reliability and lifetime of mechanical switches and the IRD's of the latter effort are more expensive and still exclude CATV signals.